Cathode sputtering is widely used for the deposition of thin layers of material onto desired substrates. A cathode assembly is mounted in a processing chamber with an anode, and the cathode assembly includes a target which is often attached to a backing plate. The chamber is evacuated and filled with an inert gas, preferably argon. A high voltage electric field is applied across the cathode and the anode. The inert gas is ionized by collision with electrons ejected from the cathode. Positively charged gas ions are attracted to the cathode and, upon impingement with the target surface, dislodge the target material. The dislodged target material traverses the evacuated chamber and is deposited as a thin film on the desired substrate, for example, a wafer, which is located close to the anode.
Referring to FIG. 2, a backing plate-target assembly 10 includes a backing plate 12 upon which is attached a target 14. The target can be fabricated from one of many different materials, for example, silicon, tungsten titanium, nickel, iron, chromium, indium oxide-tin oxide or cobalt, etc. The backing plate-target assembly 10 is attached to a cooling jacket 16 which confines a liquid coolant, for example, water. The coolant enters through a port 20 and flows across the rear surface 22 of the backing plate 12 to extract the heat generated in the target 14 by the sputtering process. The coolant exits through a port 21. The backing plate 12 provides a structural support for the target 14 during handling and use. The backing plate 12 also provides structural support and resistance to bending from both coolant and atmospheric pressure and from thermal stresses imposed by the sputtering process. The backing plate 12 must also provide a thermal path to transfer the heat from the target 14 which is generated by the sputtering process. Several different bonding processes may be used to attach the target 14 to the backing plate 12. Of current interest is the often used process of bonding the target 14 to the backing plate 12 using a low melting point solder alloy which forms a bonding layer 26 between the backing plate 12 and the target 14.
The soldered target and backing plate assembly is then installed in the cathode assembly of the processing chamber. The target is consumed by the sputtering process, and thereafter, the backing plate-spent target assembly must be removed and replaced with a new backing plate-target assembly. In continuous production, the backing plate-target assembly may require replacement after only two days of use. Often, sputtering systems contain four or more cathode assemblies in areas of restricted accessibility. Further, the sputtering system is generally operated in an ultra-clean manufacturing area which precludes the use of mechanical aids such as hoists or forklifts. Therefore, the backing plate-target assembly must be handled manually, and the weight of the backing plate-target assembly is a significant factor in the convenience and speed of the exchange of backing plate-target assemblies in a cathode assembly.
The sputtering process requires that the backing plate have certain properties. First, it should be capable of structurally supporting the target. Second, it should conduct electrical power to the target efficiently, that is, with a low resistance to minimize power loss. Third, the backing plate should be an effective thermal conductor to dissipate and transfer heat from the target. Fourth, the front surface of the backing plate should be readily bondable to numerous different target materials using a low melting point alloy, for example, tin and lead or tin and indium. Fifth, the rear surface of the backing plate exposed to the recirculating coolant should be corrosion resistant. In addition, the backing plate should be easily machinable and reasonable in cost.
A conventional material which provides all of those properties is copper, and copper is often used as a backing plate material to which the target is attached by soldering. The front surface of a copper backing plate is easily wettable with solder and will readily bond solderable target materials. Target materials that are not readily solderable will often have one or more layers of nickel sputtered or plated onto to their backside, and a copper backing plate can be readily soldered thereto. In addition to having all of the above properties, copper is very compatible for use in a vacuum and has sufficient strength and stiffness to avoid bending from either general handling, the thermal stresses of the sputtering process, or the pressure forces of the coolant. Therefore, historically, copper has been considered an almost ideal backing plate material.
There is a continuing effort to improve the yields and efficiencies of the sputtering process, and therefore, over time, the size of the wafer substrates being processed has continuously increased. Currently, wafers of up to 12 in. diameter are being processed, and their size will certainly increase in the future. To accommodate a 12 in. diameter wafer, a backing plate-target assembly must be approximately 18 in. in diameter. To maintain the desired structural integrity, the copper backing plate may be approximately 0.75 in. thick; and given a copper density of approximately 0.323 lb/in.sup.3 the copper backing plate will weigh up to 60 pounds. Clearly, in an environment where the backing plate and target assembly must be changed often and generally manually, without the aid of mechanical equipment, a weight of sixty pounds is a severe impediment. Therefore, there is a need for a backing plate that has the desirable properties of copper but does not have the undesirable property of being heavy. There is a need for a light weight copper-like backing plate that can be more easily manually manipulated in and out of the sputtering chamber.